# Energy required to turn seawater into drinking waterJuly 27, 2011 2:30 AM   Subscribe

Assuming an island with its own supply of power via generator and/or solar power, how much electricity is consumed to turn N liters of sea water into potable drinking water?

My question is as follows. Imagine you have an island such as one of the smaller San Juan islands in Puget sound (the ones where 100% of the land is privately owned and there is no regular ferry service). On this island is a house with a very large off grid solar setup capable of producing 3500 to 5000 kWh per month in mid summer.

For the purpose of this question, the argument has no reliable fresh water supply other than what can be captured from rain and run through filters.

Using a reverse osmosis process how much energy is required to desalinate seawater to turn it into drinking water? Specifically, how much energy (in watts) at a point in time to run a small reverse osmosis plant that produces N liters per hour, and if you want to produce X liters per month, how many kWh (kilowatt-hours) of electricity will that consume in one month?
posted by thewalrus to Technology (11 answers total) 2 users marked this as a favorite

According to Energy Requirements of Desalination Processes, the best you'll get is about 5 kWh for a cubic meter of fresh water.

The theoretical maximum is 0.86 kWh per cubic meter.
posted by vacapinta at 2:51 AM on July 27, 2011 [1 favorite]

For reverse osmosis desalination the figure is around about 2½ kWh m−3.

Asking "how much energy (in watts)" is a nonsensical question, as energy is measured in joules (1 kWh = 3600000 J). It is power (the rate of "using" energy) that is measured in watts.

For example: say you want to produce 1m3 (1000 litres) of water per hour. That will require 2.5 kWh of energy which is equal to nine million joules. This nine million joules of energy will be used in 3600 seconds so the power is 9000000/3600 = 2500 watts.
posted by alby at 3:50 AM on July 27, 2011

Output (L) 25.4 l/h (± 15%)

Power requirements 12-volts DC/18 amps
posted by jannw at 4:09 AM on July 27, 2011

Well depending on if they have storage or not for the pv array, the draw (in watts) may not be such a senseless question. That said i think you should be able to scale the size of your pump to your average output.

It looks like you need like 600-1200 psi of pressure (alot) for seawater. Its just a matter of optimizing a pump size with power available.
posted by ihadapony at 4:13 AM on July 27, 2011

and if they have enough sun for solar power, would some passive evaporative/condensing solution be better than reverse osmosis etc ?
posted by k5.user at 7:19 AM on July 27, 2011 [1 favorite]

You want the slingshot!

It pulls 500 watts and produces 10 gallons per hour. There is an electrical generator designed to power the slingshot (and the village that uses it!) that can be run off, among other things, methane from cow dung.
posted by jsturgill at 7:38 AM on July 27, 2011

Then there's also the consequence that they need to get rid of the salt. Dumping it right back into the water will kill off local life. :*(
posted by zombieApoc at 7:49 AM on July 27, 2011

Then there's also the consequence that they need to get rid of the salt. Dumping it right back into the water will kill off local life. :*(

Probably not at this scale, though hypersaline plumes are a big problem with utility scale desalination plants.
posted by atrazine at 8:31 AM on July 27, 2011

Then there's also the consequence that they need to get rid of the salt. Dumping it right back into the water will kill off local life.

The salt could refined for the boutique sea salt market.
posted by Horselover Phattie at 9:55 AM on July 27, 2011

Seeing as how you only need the water during the dry summers you will have abundant sunlight i would go with a solar still setup and build a big sand filter using 55 gallon drums. There are lots of plans online to build each one and the material cost is not too bad. You might have to run the water through a still a few times to get all the salt out and the filter will kill any nasties and remove the bad taste (it may not be necessary at all. The best part is all the free sea salt you will get that you can use for fun things like making bacon and salting salmon...

Another possible solution that might be cheaper than a desalination machine is a very large cistern (or several) for your freshwater needs that will last through the dry season. You can always increase your catchment area with tarps or such to fill up more cisterns if you need to.
posted by bartonlong at 10:54 AM on July 27, 2011

You might have to run the water through a still a few times to get all the salt out

If once though a still doesn't remove all the salt, you're doing it wrong.
posted by HiroProtagonist at 9:35 PM on July 27, 2011

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